Technical Field
The present invention is related to a projection zoom lens, and particularly to a projection zoom lens for use in a projection type display device.
In addition, the present invention is related to a projection type display device equipped with a projection zoom lens.
Background Art
Conventionally, projection type display devices that utilize light valves, such as liquid crystal display elements, DMD's (Digital Micromirror Devices: registered trademark), etc., are in wide use. In addition, projection type display devices which are capable of displaying higher resolution images applicable to large screens are also being utilized recently, in cinema theaters and the like.
The three plate method, in which three light valves are provided, each for a primary color, a light beam from a light source is separated into three primary colors with a color separating optical system, combined by a color combining optical system after passing through each of the light valves, and then projected, is adopted in the aforementioned projection type display devices which are utilized in cinema theaters and the like. Therefore, there is demand for a long back focus and favorably telecentric properties in such projection type display devices.
In addition, zoom lenses having high zoom ratios are desired as projection lenses in projection type display devices which are provided for use in cinema theaters and the like, so as to be compatible with projection distances and screen sizes which differ in each theater and each hall, and further the aspect ratio (cinema scope size, vista size, etc.) of displayed images, in order to match the sizes of displayed images with screen sizes.
There is also often demand for the numerical aperture (hereinafter, also referred to as “F number”) of these types of zoom lenses to be maintained constant throughout the entire zoom range, in order to prevent the brightness of displayed images from changing when magnification is changed in these projection zoom lenses in order to match the sizes of displayed images with screen sizes.
Further, projection type display devices are becoming compact and available at low cost, accompanying the digitization of cinema screens. There is a tendency for miniaturization and cost reduction to be desired in projection zoom lenses, in addition to the aforementioned demand for back focus, telecentric properties, and high zoom ratios.
Japanese Unexamined Patent Publication Nos. 2006-184723, 2005-106948, and 2008-046259 disclose zoom lenses which are presumed to be applied to projection type display devices More specifically, Japanese Unexamined Patent Publication No. 2006-184723 discloses a projection zoom lens having a six group configuration that includes a first lens group having a negative refractive power provided at the most magnification side and a final lens group having a positive refractive power provided at the most reduction side, which are fixed when changing magnification. An aperture stop is provided in a fourth lens group in this zoom lens. Meanwhile, Japanese Unexamined Patent Publication No. 2005-106948 discloses a projection zoom lens that maintains a constant numerical aperture, by configuring lens groups toward the reduction side of an aperture stop to not move while changing magnification. In addition, Japanese Unexamined Patent Publication No. 2008-046259 discloses a projection zoom lens that includes a first lens group having a negative refractive power provided at the most magnification side and a final lens group having a positive refractive power provided at the most reduction side, which are fixed when changing magnification. The numerical aperture of this projection zoom lens is maintained constant throughout the entire zoom range, by a variable stop, of which the aperture diameter varies accompanying changes in magnification, being employed.